EDM apparatuses that use an electrically-conductive traveling wire for an electrode first generally became commercially available in approximately the 1970's. These apparatuses are commonly presently used in making precise cuts and shaping various electrically-conductive materials or workpieces. Typically, the wire electrode is wound around and travels between two turning spools and a pair of wire guides or guide assemblies, wherethrough the wire travels, are used to accurately situate or guide the moving wire electrode at the proper location with respect to the workpiece being cut. The wire and the workpiece are placed at different electrical potentials and a controlled electrical spark traveling between the wire and the workpiece causes the workpiece to be eroded and cut or shaped in a desired manner.
The first wire EDM apparatuses utilized a positioning table capable of moving the workpiece in a single plane along two orthogonal axes. By providing numerically-controlled servo motors on each of the two axes, the workpiece could be positioned with respect to the wire electrode for producing perpendicular cuts as desired along the workpiece.
Wire electrode machines were, thereafter, modified to provide taper cutting. Initially, the upper wire guide assembly was orbited to produce a taper cut such as is needed, for example, for draft clearances between punch and dies. Although orbiting the upper guide assembly worked for providing small angled cuts, the cutting speed was severely decreased because more material had to be removed. Further, the taper was limited to the offset in the oscillating guide.
Steeper angular cuts were later made capable without orbiting the upper wire guide but, rather, moving the upper guide with respect to the lower guide and placing the wire electrode at an angle with respect to the workpiece. As can be appreciated, although this means of angle cutting is effective, it is limited to smaller angles because of the inaccuracies created by the path of the wire being distorted. This distortion is shown in FIG. 1A whereat the upper and lower guides G are offset from one another and the guide outer surface S causes the wire W to be moved at an angle .alpha. rather than .beta. of the line intersecting reference points R. In addition, the wire W is caused to distort and overshoot the desired path and further inaccuracies are thus created.
A solution to EDM angle cutting has been to mechanically pivot the upper and lower wire guide assemblies while simultaneously moving the assemblies in a plane parallel to the workpiece and, thus, having the wire electrode exit and enter the guide assemblies generally co-linear therewith. Examples of such wire guiding assemblies and apparatuses are shown in U.S. Pat. Nos. 4,667,078 and 4,751,362. As can be appreciated, although these guide assemblies provide for angle cutting, the mechanical components for pivoting the guide assemblies create a system that is very complex, difficult to maintain, and generally costly.
Accordingly, a need exists for a wire electrode guiding device that solves the problem associated with prior guiding devices and provides for EDM larger angle cutting with a generally dependable, lower cost and accurate system.